1. Field of the Invention
This invention relates to a distance measuring device for measuring a distance from the device to an object by using a laser beam or an electromagnetic wave, and more particularly to an improved device designed for a safety operation.
2. Discussion of the Related Art
Referring to FIG. 11, there is shown a block diagram of a conventional distance measuring device employing a laser beam. In accordance with a clock generated from a controller 7, a laser diode (LD) 2 is energized to emit laser beams by a laser diode (LD) driver 3. The laser beams as a kind of electromagnetic waves are sequentially changed to a predetermined directions by a scanner 1 associated with controller 7. Thus emitted laser beams are partially reflected by an object (omitted in the drawing) to be received by a photodiode (PD) 5 for application to controller 7 through a light receiving circuit 6. A scanning position detector 4 is designed to detect a scanning direction and a scanning angular speed of scanner 1 to be applied to controller 7.
Scanner 1 for sequentially changing the emitted beams into the predetermined directions will be described hereinafter in detail in reference to FIG. 12.
In FIG. 12, beams emitted from laser diode 2 are changed to parallel beams P by a collimator lens 9. The parallel beams P are changed to a predetermined direction by a stationary mirror 11 and a scanning mirror 13 for emission. Scanning mirror 13 is adapted to be driven by a motor (omitted in drawings) for sequential scanning directions.
The scanning direction of the radiated parallel beams P can be detected by the scanning position detector having a scanning control laser diode 15 and a position sensing diode (PSD) 17. The scanning direction can be detected by finding a position of position sensing diode 17 which receives a beam emitted from scanning control diode 15 through scanning mirror 13 of a both side mirror type.
As shown In FIG. 13, the emitted beams from scanner 1 are reflected by an object 19. When the reflected beams are received, an elapsed time T1 from emission to receipt is calculated by controller 7. At the same time, the scanning direction (angular speed) can be detected. In view of these data, a distance calculation circuit 8 of FIG. 11 calculates the following distance. A linear distance D from scanner 1 to object 19 can be obtained by multiplying a speed of light by an elapsed time and dividing by two as shown by an equation: D=T1.times.299792458/2 m/s!. The vertical distance Dc and the parallel distance Ds can be obtained by the following equations . EQU Dc=D.times.COS .theta. m/s! EQU Ds=D.times.SIN .theta. m/s!
Thus distance measuring device may be employed by radars or vehicles. For example, by installing the device around a bumper of an automobile, a distance from the automobile to its neighbor vehicles can be obtained. The distance between two cars can be obtained by the vertical distance Dc. The existence of a vehicle in its neighbor lane or a distance to the neighbor vehicle therefrom can be found by a parallel distance Ds.
The conventional distance measuring device has several disadvantages. The emitted laser beams are sequentially radiated to predetermined scanning directions by scanner 1. FIG. 14 shows a scanning field 27 between scanning start point and scanning end point. The change of scanning direction is done by the rotation of scanning mirror driven by motor, whereby the angular speeds around scanning start point 21 and scanning end point 23 are accompanied with acceleration or deceleration. The angular speeds around points 21 and 23 are stow in comparison with speeds in other scanning field 25. Since the emission is continuously executed, quantity of light per a unit area becomes increased when the angular speed of the scanning mirror is decreased. The Increased quantity of beams is danger when it hits human eyes, and unfavorable in view of safety control.